A plurality of switched mode power supply topologies are known to the skilled person including the buck converter, the boost converter, the fly back converter, the buck boost converter and others. These circuits convert a DC input voltage into a desired output voltage by using at least one main switching element (which may be realized in a plurality of ways, as known to the skilled person, e.g. as a field effect transistor FET) and least one reactive element (i.e. a circuit element which can at least temporarily store energy, usually implemented as an inductor). In operation, the main switching element is continuously switched in a controlled manner, so that the input voltage is converted into an output voltage of desired voltage level.
It is known to use a switched mode power supply to simultaneously drive a plurality of loads. These loads may be connected to the power supply e.g. in parallel fashion.
An example of an application where multiple loads are to be supplied are lighting and display applications e.g. for video screens and projectors, that employ multiple light sources. These light sources, which are connected to the power supply circuit as loads may be e.g. LED, OLED, or laser diodes, e.g. of different color. It should be noted that in the present context the light sources may be single elements, such as single LEDs, but that the term “light source” is also used for any array of single lighting elements, which may be connected e.g. in series or parallel. It is known to provide accurate dimming of the light sources by driving them in a pulsed manner. However, power supply circuits suited for use in display applications need to provide quite exact pulse shapes in order to obtain a correspondingly exact result. This is especially true for time-sequential display applications, where the light sources are driven rapidly in time-sequential manner to achieve color and intensity control, and/or the light is additionally modulated by a display device.
For driving light sources in a pulsed manner, the required voltage or current (pulse height) may be fixed for one light source, but will differ between different light sources, e.g. of different color. Also, for some display applications it is known to drive the same light source sequentially with pulses of different height (i.e. voltage or current level).
To drive the loads as described above, it is possible to provide a single, dedicated switched mode power supply for each load and each required voltage output level. However, the corresponding outlay is enormous.
WO-A-2007-039862 describes a driver circuit arrangement for driving a plurality of individually switchable electrical subsystems, such as arrangements of LEDs. The subsystems are all connected in parallel to the same switched mode power converter. Each subsystem is comprised of an LED (the load) connected in parallel to a capacitor with a controllable subswitch for connecting the load to the capacitor. Further, each subsystem comprises a load switch which connects or disconnects the subsystem to/from the power converter. In operation, an external control unit selects the subsystems to be supplied with energy by closing the corresponding load switch. In contrast to prior circuits which do not have a subswitch between the capacitor and the load, it is then possible to control the load, e.g. in pulse width modulation mode by controlling the subswitch independently from the load switch.
If multiple loads requiring different voltage levels are driven by the same switched mode power supply circuit, or if at least one load requires different voltage levels at different times, then the problem of voltage mismatch may occur: after supplying a first voltage level for a first pulse, the circuit will require some time to supply a different voltage level for a second, following pulse. This will lead to distortion of the pulse shape and subsequently worsened control results.